An organic electroluminescent device (hereinafter, “electroluminescent” is often abbreviated as “EL”) is a self-emission device utilizing the principle that a fluorescent material emits light by the recombination energy of holes injected from an anode and electrons injected from a cathode when an electric field is applied.
Since C. W. Tang et al. of Eastman Kodak Co. reported a low-voltage driven organic EL device of stacked type (non-Patent Document 1, for example), studies on organic EL devices in which organic materials are used as constitution materials has been actively made.
The organic EL device reported by Tang et al. has a stacked structure in which tris(8-hydroxyquinolinol)aluminum is used as an emitting layer and a triphenyldiamine derivative is used as a hole transporting layer. The advantage of the stacked structure include increased injection efficiency of holes to the emitting layer, increased generation efficiency of excitons generated by recombination while blocking electrons injected from the cathode, and containing the excitons generated in the emitting layer.
As the stacking structure of the organic EL device, a two-layer type formed of a hole transporting (injecting) layer and an electron-transmitting layer, or a three-layer type formed of a hole transporting (injecting) layer, an emitting layer, and an electron transporting (injecting) layer or the like is well known. In such a device with a stacked structure, the device structures or the fabrication methods have been contrived to increase recombination efficiency of injected holes and electrons.
Heretofore, an aromatic diamine derivative as described in Patent Document 1 or an aromatic condensed ring diamine derivative as described in Patent Document 2 is known as a hole transporting material used in an organic EL device.
In an organic EL device using the aromatic diamine derivative as the hole transporting material, a high applied voltage is required in order to obtain a sufficient luminance. Applying a high voltage causes such problems as shortened lifetime of the device, increased power consumption, and the like.
To solve the problems, doping a hole-injection layer with an electron-receiving compound such as Lewis acid or the like has been proposed (Patent Documents 3 to 6, or the like) However, the electron-receiving compounds used in those Patent Documents have disadvantages that they are unstable to handle during fabricating an organic EL device, that the lifetime of an organic EL device fabricated using these compounds is shortened due to a lowering in stability such as heat resistance when an organic EL device is driven, and the like.
Tetrafluorodicyanoquinodimethane of an electron-receiving compound described in Patent Documents 5, 7, 8 and the like is sublimed readily since it has a low molecular weight and is substituted with fluorine. Therefore, tetrafluorodicyanoquinodimethane may diffuse within an apparatus when fabricating an organic EL device by vacuum deposition, causing the apparatus or the device to be contaminated. In addition, it is crystallized when forming a device therefrom to cause current leakage.
The inventors focused attention on an imide derivative and made extensive studies. In more detail, the inventors focused attention on naphthalene tetracarboxylic acid diimide derivatives and pyromellitic acid diimide derivatives. It is known that these compounds form charge transfer complexes with amine derivatives of donor compounds (Non-patent Document 2). These compounds are also known as a material for an electrophotographic photoreceptor (Patent Document 9).
Since the above-mentioned imide derivatives have electron receiving properties and excel in heat resistance, it is expected that the productivity is not decreased due to decomposition thereof during depositing, and the resulting EL device does not deteriorate due to Joule heat and the like which occurs during driving of the device.
However, a reduction potential of the imide derivatives, which are used for an electron transporting material of an electrophotographic photoreceptor, is −1.5 to −0.5 V (vs saturated calomel electrode, and “vs saturated calomel electrode” is often abbreviated as “vs SCE” hereinafter), and the electron receiving properties thereof are weak. Therefore, the imide derivatives have an insufficient performance so that they cannot be used for an organic EL device.    [Patent Document 1] U.S. Pat. No. 4,720,432    [Patent Document 2] U.S. Pat. No. 5,061,569    [Patent Document 3] JP-A-2003-031365    [Patent Document 4] JP-A-2001-297883    [Patent Document 5] JP-A-2000-196140    [Patent Document 6] JP-A-11-251067    [Patent Document 7] JP-A-4-297076    [Patent Document 8] JP-T-2004-514257    [Patent Document 9] JP-A-2001-040237    [Non-patent document 1] C. W. Tang, S. A. Vanslyke, Applied Physics Letters, 51, 913 (1987)    [Non-patent document 2] Japan Polyimide Forum, Latest Polyimide, NTS inc. and the like
The invention has been made based on the above problems. An object of the invention is to provide an electron-receiving material suitable as a constitution material of an organic EL device.
An object of the invention is to provide an organic EL device which can be driven at a low voltage and have a long lifetime.
The inventors made extensive studies and found that a novel imide compound or a specific imide derivative, which is derived from a pyromellitic acid of electron-receiving compound and has an electron-withdrawing group, has high electron-receiving properties, and improved heat resistance and electrical characteristics. The inventors found that an organic EL device using this imide derivative as an organic EL device material can be driven at a low voltage and can exhibit a long lifetime.
The invention provides the following imide derivative, and the like.    1. An imide derivative represented by the following formula (A):
     wherein Ra and Rb are each a hydrogen atom, a halogen atom, a cyano group, an alkyl group, a fluoroalkyl group or an aryl group; at least one of Ra and Rb is a fluoroalkyl group; and Rc and Rd are each a substituted or unsubstituted benzyl group, an aryl group, a heterocycle, a fluoroalkyl group or an imide group.    2. A material for an organic electroluminescent device represented by the following formula (I):
     wherein R1 to R10 are each a hydrogen atom, a halogen atom, a fluoroalkyl group or a cyano group, provided that a material wherein all of R1 to R10 are a hydrogen atom is excluded.    3. A material for an organic electroluminescent device represented by the following formula (II):
     wherein R11 to R20 are each a hydrogen atom, a halogen atom, a fluoroalkyl group or a cyano group, provided that a material wherein all of R11 to R20 are a hydrogen atom is excluded.    4. The material for an organic electroluminescent device according to any one of 1 to 3 which have a reduction potential (vs saturated calomel electrode) in a dimethylformamide solution of −0.5 V or more.    5. An organic electroluminescent device comprising:
an anode,
a cathode, and
one or a plurality of organic thin layers, including an emitting layer, the organic thin layers being interposed between the anode and the cathode;
at least one of the organic thin layers containing the imide derivative of claim 1 or the material for an organic electroluminescent device of any one of 1 to 4.    6. The organic electroluminescent device according to 5 wherein the organic thin layers are a multilayer body in which a hole transporting layer, an emitting layer and an electron transporting layer are stacked in this order from the anode.    7. The organic electroluminescent device according to 6 wherein the hole transporting layer contains the imide derivative or the material for an organic electroluminescent device.    8. The organic electroluminescent device according to 5 wherein the organic thin layers are a multilayer body in which a hole injecting layer, a hole transporting layer, an emitting layer, and an electron transporting layer are stacked in this order from the anode; the hole injection layer containing the imide derivative or the material for an organic electroluminescent device.    9. The organic electroluminescent device according to 7 or 8 wherein the hole transporting layer or the hole injecting layer, the hole transporting layer and the hole injecting layer containing the imide derivative or the material for an organic electroluminescent device, further contains a phenylenediamine compound represented by the following formula (III):
     wherein R21 to R26 are a hydrogen atom, a halogen atom, a trifluoromethyl group, an alkyl group, an aryl group, or a heterocycle; R21 to R26 may form a naphthalene skeleton, a carbazole skeleton, or a fluorene skeleton with a phenyl group bonded; and n represents 1 or 2.
According to the invention, a novel material for an organic EL device is provided. Also, according to the invention, an organic EL device which can be driven at a low voltage and has a long lifetime is provided.